Chemical mechanical polishing is a technique that is well known, used both in optics and in microelectronics. Its principle consists in pressing the surface to be polished with force against a polishing element that is in motion relative thereto and that is soaked in a suspension of abrasive particles known as slurry. The polishing element is typically a pad of polyurethane foam or a felt of textile fibers bonded together by a polyurethane matrix. By way of example, the slurry may be colloidal silica, a suspension of cerium oxide, etc.
More detailed information on this technique is to be found in the PhD thesis of Jiun-Yu Lai “Mechanics, mechanisms, and modeling of the chemical mechanical polishing process”, Massachusetts Institute of Technology, Feb. 2001.
In its most common form (rotary CMP) the polishing element is circular in shape and performs rotary motion; a “workpiece-carrier” keeps the workpiece that is to be machined rotating with one of its surfaces in contact with the polishing element. There are also exist linear CMP machines in which the polishing element is carried by a looped belt driven with linear motion, like a conveyor belt. Only rotary CMP is considered in detail below, but the invention is equally applicable to linear CMP.
When it is necessary to polish both faces of a workpiece, such as a lithographic mask, it is advantageous to use a dual-face CMP method in which the workpiece is sandwiched between two polishing elements that apply a compression force. The workpiece-carrier must be designed to allow both faces to make contact simultaneously with the polishing elements.
Experience shows that the edge of the workpiece presents over-polishing that can be very considerable. This is due to the polishing element being flattened, giving rise to extra pressure in the vicinity of said edge, and also to abrasive particles accumulating. This results in a non-planar zone in the polished surface that can extend over a significant fraction of the diameter of the workpiece, for example over about 15 mm for a workpiece that is 150 mm in diameter (10%). The effect is even more marked for non-circular workpieces presenting sharp angles. A more thorough discussion about the effect of over-polishing is to be found in the article by Jianfeng Luo “Wafer-scale CMP modeling of within wafer non-uniformity”, Laboratory for Manufacturing Automation, University of California, Berkeley.
A first solution to this problem consists in providing workpieces with a peripheral zone that is to be cut off after polishing. Apart from the fact that that technique is very expensive and involves wasting material, the cutting operation itself induces mechanical defects that degrade the surface state of the workpiece. It is therefore not adapted to lithographic masks, and more generally to ultraviolet optics, since the maximum size of defects that can be accepted is no greater than a few tens of nanometers.
A second solution, e.g. as described in the above-mentioned article by Jianfeng Luo, consists in surrounding the workpiece with a guard ring, and it is the guard ring that is subjected to over-polishing instead of the workpiece. That technique is also expensive since the guard ring needs to be produced with tolerances that are very strict and it needs to be replaced after a small number of uses. This drawback is particularly marked with dual-face polishing since the ring must have exactly the same thickness as the workpiece and a single use can thin it to such an extent as to make its replacement necessary.